1. Technical Field
This invention pertains generally to the field of wireless cellular communications. More particularly, the invention describes a method and apparatus for automatically diagnosing the backhaul link of a cellular system that uses RF repeaters.
2. Description of the Related Art
Conventional wireless cellular communications systems have a common architecture in which one or more defined cell sites are formed by the placement of one or more base transceiver stations within a geographic area. A cell site is typically depicted as a hexagonal area in which a transceiver is located such that a radio communication link can be established between the cellular system and a plurality of mobile stations within the defined geographic area for the cell.
In order to extend the coverage of conventional base transceiver station (BTS) systems over a larger geographical area, cell service providers have found it useful to employ translating repeaters. In the uplink direction, signals transmitted by a mobile station (mobile transceiver unit) located in a remote cell are received at the repeater, translated to a different carrier frequency, and then transmitted to the host BTS. Likewise, in the downlink direction, signals transmitted by the host BTS are received by the repeater, translated to a different carrier frequency, and then transmitted to mobile stations. The RF carrier link between the repeater and the BTS is known as the xe2x80x9cbackhaul channel,xe2x80x9d hereinafter, backhaul channel, and the carrier frequency on which the backhaul channel is operated is called the xe2x80x9cbackhaul frequency.xe2x80x9d
Some translating repeaters, such as the AirSite(copyright) repeater system offered by AirNet Communications Corporation of Melbourne, Fla., advantageously make use of existing inband RF carrier frequencies to backhaul cellular communications traffic. As used herein, the term xe2x80x9cin-bandxe2x80x9d refers to carrier frequencies that are within the frequency spectrum allocation assigned to the service provider for providing cellular communications services to mobile subscribers. Use of in-band radio frequency channels to backhaul cellular communications traffic from remote repeater sites is highly advantageous as it eliminates costly wireline T1 or microwave connections.
Interference on the backhaul communications link can be caused by a variety of sources. As the number of subscribers on a cellular system grows, new equipment must be added in order to accommodate the increased usage. The addition of new repeaters in a repeater based cellular communications system can affect the performance of the backhaul communications link. In general, the link conditions can deteriorate due to congestion as the link reaches its full capacity. In addition to more traffic traversing the link, differences in tolerances between the various network entities accessing the bus can also degrade the backhaul communications link.
Interference on the backhaul link can be a problem for several reasons. For example, since the repeater station recovers its clock from the downlink channel of the backhaul communications link, the signals on the backhaul link must be maintained at a certain quality in order for the repeater station to maintain synchronization with the base station. Additionally, the slot/frame timing information for the downlink signal is derived as an offset to the uplink signal. Accordingly, interference on the backhaul communications link can adversely affect the uplink timing. Finally, the bit error rate (BER) can be adversely affected by any interference on the backhaul communications link.
The backhaul communications link can be diagnosed in a variety of ways. For example, a technician can use test equipment to determine the conditions that exist on the communications link. Alternately, loopback testing for diagnosing the link can be used. However, these systems are not completely satisfactory for testing the backhaul communication link of a repeater based system. For example, manual testing can be expensive and time consuming. By comparison, loopback systems are more convenient and less expensive, but are best used when testing the complete transmit and receive communication path through which a signal must travel.
In particular, in order to accurately test the conditions on the RF channel comprising the backhaul communication link, the uplink and downlink transmission paths on the link must be tested independent of the internal path of the repeater system. If a loopback test was employed to test the backhaul communication, then the link statistics would be corrupted by any processing internal to the repeater system. For this reason, loopback systems suffer certain drawbacks for testing the backhaul communication link.
The invention concerns a method for diagnosing a backhaul communication link of a repeater based wireless communication system. The wireless communication system has a base station located within a home cell, and a plurality of substantially adjacent cells, at least one of the plurality of cells having a repeater station located therein. The method comprises automatically measuring an interference level for the backhaul communication link and responsive to the measured interference level, selectively modifying the operation of the backhaul link to overcome any adverse effect of the interference level. The modifying step can comprise one or more of increasing a power level of signals transmitted over the backhaul link, decreasing the power level of signals transmitted over the backhaul link, assigning an alternative frequency for use as the backhaul link, or sending a message to an operation maintenance center.
The measuring step can comprise automatically measuring the interference level on an RF carrier frequency of a backhaul channel assigned for the backhaul communications link. The measuring step can also include automatically measuring the interference level on one or more RF channels adjacent to the RF carrier frequency of the backhaul channel. Further, the measuring step can include disabling signal transmissions on the backhaul channel and measuring signal levels of noise or interfering signals occurring on the RF carrier frequency of the backhaul channel. Finally, the measuring step further comprises transmitting a test signal on the RF carrier frequency of the backhaul channel and measuring an adverse effect of one or more of a noise level and an interfering signal level on the test signal. The adverse effects of noise or interference on the channel can be measured by determining at least one of a carrier-to-noise level, a carrier-to-interference level, a bit error rate, or a block error rate. The measuring step can be performed at predetermined intervals, upon detection of a predetermined number of detected bit errors, upon detection of a predetermined number of detected block errors, or upon detection of a predetermined number of frame erasures.
In a further embodiment of the invention, a system is provided for diagnosing a backhaul communication link of a repeater-based wireless communication system. The system includes a base station located within a home cell, and a plurality of substantially adjacent cells, one or more of which can include a repeater station located therein. The system comprises circuitry and/or software for automatically measuring an interference level for the backhaul communication link. Such measurements can include automatically measuring the interference level on an RF carrier frequency defining a backhaul channel assigned for the backhaul communications link and automatically measuring the interference level on at least one RF channel adjacent to the backhaul channel. The measurements can also include disabling signal transmissions on the backhaul channel and measuring at least one of a noise level and an interfering signal level. Finally, the measurements can include transmitting a test signal on the backhaul channel and measuring any adverse effect of noise or interfering signals on the test signal. Control circuitry and software is also provided so that a transmitter connected to the backhaul communications link can be selectively controlled to modify the operation of the backhaul communication link to overcome any adverse effect of the interference level. This modification can include an increase in power level of signals transmitted over the backhaul link, a decrease in power level of signals transmitted over the backhaul link, the assignment of an alternative frequency for use as the backhaul link, and/or a message being sent to an operation maintenance center.